In the following description, specific reference is made purely by way of example to the cutting or embossing of strip paper on automatic packing machines.
Automatic packing machines are known to feature cutting or embossing units comprising two mutually-cooperating rollers, which are fitted to respective supports, rotate about respective substantially parallel axes, and define a work region to which the sheet material is fed for processing by a pair of mutually-cooperating tools, each fitted to a respective roller.
Optimum performance of the tools, in terms of quality processing of the material and minimum tool wear, depends on the way in which the tools mate, i.e. on the tools cooperating mutually according to a given law of interaction, in turn, depending on the spatial relationship of the two rollers. For example; two cutting rollers fitted with respective numbers of blades cooperating in pairs operate best when the blades in each pair skim over each other with no interference.
An error or shift in the spatial relationship of the axes of the two rollers results in impaired interaction of the tools and, consequently, in poor-quality work; and, especially in the case of cutting units, tool wear, i.e. of the blades, is greatly increased in the event of interference between the blades in each pair.
In the case of two rollers fitted with respective numbers of tools, the spatial relationship of the roller axes providing for optimum interaction of one pair of tools rarely also applies to the other pairs, due, for example, to the different tool assembly tolerances involved, so that setting up the processing unit is a particularly painstaking, and hence expensive, job, which invariably amounts to a trade-off between the spatial relationships of the roller axes providing for optimum working conditions of all the tool pairs.
Moreover, optimum working conditions are affected fairly rapidly by in-service slack and wear of the tools, so that the processing unit must be adjusted frequently, thus further increasing maintenance cost.
EP-A1-707928 and EP-A1-841133 (intermediate document according to Art. 54(3) EPC) disclose a rotatory cutter comprising a knife roller and a plain roller cooperating with each other, and a clearance adjusting mechanism disposed on both end portions of the two rollers for adjusting in use the contact pressure between the knife roller and the plain roller. The clearance adjusting mechanism comprises a toggle mechanism coupled with a threaded member driven by a gear box connected to an electric motor.
The aforementioned clearance adjusting mechanism has several drawbacks, which stem from the fact that such mechanism generates the movement of the two rollers mechanically, and is therefore a relatively slow and low-precision mechanism. Furthermore, the above clearance adjusting mechanism is quite expensive owing to its generating a high precision movement combined with a relatively strong force.